As is known, infrared cameras generally employ a lens working with a corresponding infrared focal plane array (FPA) to provide an image of a view in a particular axis. The operation of such cameras is generally as follows. Infrared energy is accepted via infrared optics, including the lens, and directed onto the FPA of microbolometer infrared detector elements or pixels. Each pixel responds to the heat energy received by changing its resistance value. An infrared (or thermal) image can be formed by measuring the pixels' resistances—via applying a voltage to the pixels and measuring the resulting currents or applying current to the pixels and measuring the resulting voltages. A frame of image data may, for example, be generated by scanning all the rows and columns of the FPA. A dynamic thermal image (i.e., a video representation) can be generated by repeatedly scanning the FPA to form successive frames of data. Successive frames of thermal image data are generated by repeatedly scanning the rows of the FPA; such frames are produced at a rate sufficient to generate a video representation of the thermal image data.
Individual pixels have unique response characteristics. These non-uniformities often result in fixed pattern noise. Many infrared cameras have functionality to provide the ability to correct for this. For example, some infrared cameras can automatically or manually perform offset compensation, which corrects for variations in the individual pixel responses by observing a uniform thermal scene (e.g., by placing a shutter between the optics and the array) and measuring offset correction data for each pixel which provides the desired uniform output response. These measured offset corrections are stored, then later applied in subsequent infrared measurements (e.g., with the shutter open) to correct for fixed pattern noise. Other compensations can also be applied, such as 2-point correction.
As is known, offset compensation functionality is found in most conventional infrared cameras because it leads to improved imaging capabilities. However, offset compensation can be an inconvenience to the user as it necessitates activation of the camera shutter, thereby “freezing” the camera image for a short period of time when the shutter is closed. Therefore, it is desirable to keep the period between offset compensations lengthy so as to limit the general inconvenience to the user of the camera, while still maintaining good image quality.
Temperature changes within or surrounding an infrared camera are found to result in the individual pixels further exhibiting their unique response characteristics. In particular, the change in temperature of the camera's internal components, e.g., due to self-heating or as the result of changes to the surrounding ambient temperature, leads to the individual pixels exhibiting fixed pattern noise over extended lengths of time. For example, during initial powering of an infrared camera, the internal components can be found to continue to rise in temperature for a period of time before the camera becomes thermally stable. Because of this, offset compensation is often performed at an increased frequency during such period so as to maintain good image quality from the camera. Such increased frequency of offset compensation correspondingly results in an increased frequency of shutter actuation. Consequently, there is further inconvenience for the user as the shutter is closed more often during such period.
What are needed are apparatus and systematic methods to address or overcome one or more of the limitations briefly described above with respect to offset compensation functionality in infrared imaging systems.